In the field of medicine, there are certain types of medicine such as anodynes or antitumor agents that should be infused into patients over a long period of time, and these types of medicine generally require infusions thereof into the patients in substantially constant quantities per unit time. There have been apparatuses, which use common syringes, for infusing medicine into patients in a constant quantity per unit time, and the syringes are mounted in the apparatuses so that plungers of the syringes can be gradually pushed. Due to the large sizes of the apparatuses, it is unsuitable for carrying by the patients.
There has been proposed a liquid supply apparatus capable of being carried by a patient. The conventional portable liquid supply apparatus has a structure in which a resilient bladder made of rubber material is provided in a cylindrical chamber. An inlet and an outlet are formed on the resilient bladder. The bladder inflates as medicine is infused through the inlet. The medicine gradually draws off through the outlet to which an elongated tube is attached. The medicine (for infusion) is discharged in a small amount and infused into the vein of a patient. In the liquid supply apparatus, the resilient bladder may cause the following problem. Upon manufacture of the resilient bladder, inferior goods in which the thickness of the bladder is not uniform or fine holes are formed thereon may be produced. The inferiority affects the resiliency of the bladder which in turn cannot have the desired resiliency. If this is the case, it is difficult to keep an infusion quantity per unit time constant.
There has also been proposed a liquid supply apparatus that uses a bladder and an elastic body. In the resilient bladder or elastic body, its resiliency or elasticity may vary in accordance with the quantity of the medicine contained therein. An external force (the restoring force of the elastic body) on liquid flow may also be different at early and final periods of infusion. This is because the restoring force of the elastic body varies according to the amount of its deformation.
The present inventor has proposed a liquid supply apparatus with a new structure to overcome the aforementioned problems. In the newly proposed liquid supply apparatus, a piston is pushed within a cylinder by the pressure of gas generated in the apparatus. The piston drives the supply of infusion medicine to the patient. An example of the liquid supply apparatus is disclosed in International Publication No. WO 02/11791. The liquid supply apparatus of the present inventor has a superior advantage of being easily carried, maintaining a constant quantity of infusion and the like. Nevertheless, continuous improvement will be made therein in the same manner as other superior technologies.